<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<meta name="viewport" content="width=device-width, initial-scale=1.0">
	<title>Document</title>
</head>
<body>
	<style>
		.father{
			width:500px;
			height:100vh;
			border:1px solid #0a3b98;
			position: relative;
		}
		.son{
			width:100px;
			height:40px;
			background: #f0a238;
			position: absolute;
			top:0;
			left:0;
			right:0;
			bottom:0;
			margin:auto;
		}
	</style>
	<div class="father">
		<div class="son"></div>
		<div class="a">s</div>
	</div>
</body>
</html>